Some people are disqualified from donating blood because they have diseases that are transmissible via blood. Other potential donors are disqualified because their conditions could endanger themselves.

According to the American Red Cross:

Being positive for the AIDS or hepatitis viruses rules one out as a blood donor.

Individuals who have had ear, tongue, or other body part piercing are allowed to donate blood as long as the needle used in the piercing was sterile. If it was not or if this is unknown, the potential donor must wait 12 months from the time of the piercing.

Being imprisoned rules one out as a blood donor.

Being in the US military and serving in Iraq or Afghanistan rules one out as a blood donor for one year.

A person with diabetes is allowed to donate blood. Insulin dependent diabetics are allowed to donate blood as long as their insulin syringe, if reused, is used only by them.

Being deferred from travel to the UK and Western Europe due to concerns about Mad Cow Disease rules one out as a blood donor.

Physically small people are not acceptable as blood donors as they have lower blood volumes and may not be able to safely lose a full pint of blood.

One may not donate blood while one has the flu. But one can donate blood after exposure to someone with the flu provided the potential donor feels and has no symptoms.

A minimum age limit exists as to how old a person must be in order to donate blood (usually age 17). There is no maximum age limit.

Pregnancy and recent childbirth rule one out as a blood donor. The safety of donating blood during and shortly after pregnancy has not been fully established. There may be medical risks to the mother and baby during this time.

Having high or low cholesterol does not exclude a person from donating blood.

Potential blood donors may be temporarily prevented from donating if they have a low level of iron (hematocrit) in their blood. This requirement is for the safety of the donor in order to ensure that their blood iron level remains within the normal range for a healthy adult.

For almost all cancers (such as breast, brain, prostate, and lung), a person may donate blood five years after diagnosis or date of the last surgery, last chemotherapy or last radiation treatment.

For blood cancers (such as leukemia or lymphoma), a person is not allowed to donate blood.

For non-melanoma skin cancer or a localised cancer that has not spread elsewhere, a person may give blood if the tumour has been removed and healing is complete.

If a potential donor has had malaria they cannot donate blood for 12 months. This is because the parasite that causes malaria can lay dormant in a person's system for as long as a year.

A person cannot donate blood while they are on antibiotics. This is not because of the antibiotic, but due to the presence of the illness or infection requiring the antibiotic - it may be transmitted through the blood.

Stephen Juan, Ph.D. is an anthropologist at the University of Sydney. Email your Odd Body questions to s.juan@edfac.usyd.edu.au